The present invention relates generally to a process for the delignification and bleaching of lignocellulosic pulps. More particularly, the invention relates to a photo-oxygenation process for the delignification and bleaching of lignocellulosic pulps employing electronically excited species of oxygen generated in situ.
The development and adaptation of new oxidizing agents and processes in the pulp and paper industry is of interest due to manufacturing, pulp quality and environmental considerations. In recent years the pulp and paper industry has devoted considerable effort to the development and implementation of chlorine-free processes, or in the alternative processes employing reduced amounts of chlorine. These efforts have been mounted, in large part, to comply with increasingly stringent governmental regulations dictating the reduction or elimination of pollutants in both the atmosphere and the water.
One direction taken by investigators has been to conduct extensive research into various oxygen bleaching systems. While this research has proven fruitful, as attested to by the installation of several commercial pulp bleaching facilities employing oxygen as a stage in their bleaching sequence, oxygen bleaching cannot produce pulps of sufficient brightness standing alone. Another avenue which has been explored is the use of ozone as a bleaching agent, either by itself or following an oxygen bleaching stage. While ozone is an effective bleaching agent, in that it produces pulps of high brightness and which are free of chlorine, it suffers from the infirmity of causing extensive depolymerization of cellulose at the temperatures normally employed in bleach plant operations.
Another approach taken by bleaching researchers, has been investigations into the use of singlet oxygen. The work on singlet oxygen chemistry in the pulp and paper industry can be classified as falling into one of the following categories:
1. A process for the delignification and bleaching of hardwood kraft pulp using chlorine-containing compounds in combination with oxygen-containing compounds, which, through reaction in situ give rise to gaseous chlorine compounds and oxygen in a singlet or high energy state. (Berge et al., ATIP Rev., 30, No. 5, 161-166 (1976); French Pat. No. 2,255,418.)
2. Liebergott's process disclosed in U.S. Pat. No. 3,806,404 for the delignification and bleaching of chemical and mechanical pulp using active or high-energy gases generated by passing the gas through a Corona discharge and subsequently mixing the gas with pulp at high consistency.
3. Studies made with pure model lignin compounds using singlet oxygen generated via photo-oxygenation to gain a better understanding of factors influencing color reversion of cellulosics. Some of these studies suggest that singlet oxygen attack on the polysaccharide is not an important factor in cellulose photo-degradation. (Gellerstedt et al., Svensk Papperstid., 80, No. 1, 15-21 (1977); Gellerstedt et al., ACTA Chem. Scand., 29B, No. 10, 1005-1010 (1975); Carlsson et al., J. Polymer Sci., (B. Polymer Letters), 14, No. 8, 493-498 (1976); Gellerstedt et al., "Singlet Oxygen Oxidation of Lignin Structures", Canadian Wood Chemical Symp. (Mont Gabriel, Quebec). Extended Abstr. (SPPA, Montreal): 21-24 (Sept. 1-3, 1976); Meshitsuka et al., TAPPI 59, No. 11 (1976); Knubben, Auslegeschrift, German Pat. No. P 27 11 900.2-45).
4. Processes for the delignification and bleaching of lignocellulosic pulps using chlorine compounds in the presence of ultraviolet light. (Markham, TAPPI, 60, No. 9 (1977); U.S. Pat. No. 2,161,045).
5. Processes for bleaching and sterilization of a pulp web using ultraviolet light and a subsequent application of ozone. (U.S. Pat. Nos. 1,582,677 and 1,850,808).
The Liebergott et al., patent referred to above, U.S. Pat. No. 3,806,404, discloses the activation of various gases, including oxygen, by passage of such gases through a Corona discharge and subsequently treating fluffed softwood pulp at a consistency of 15% to 95% with the activated gas, resulting in the delignification and bleaching of chemical and mechanical pulps. The use of electronically excited states of oxygen with softwood is disclosed by Liebergott in Example 1(1), but the data in Table I in Liebergott indicate that activated oxygen was only marginally effective in delignifying and bleaching lignocellulosic pulps. Reference to Table I of the Liebergott patent indicates that subsequent to bleaching the Kappa number of the pulp was 22.6, which represents a reduction of only 1.4 units, which translates into a percentage reduction of only 5.8%. Certainly, careful study of the entire Liebergott patent would leave one skilled in the art with the realization that activated nitrogen was found by Liebergott to be extremely effective, while activated oxygen, under the same reaction conditions, was found to be ineffective.